The present invention relates generally to displays, and more particularly, using pulse-width modulation to drive one or more display elements of an electro-optical display.
Pulse-width modulation (PWM) has been employed to drive liquid crystal (LC) displays. A pulse-width modulation scheme may control displays, including emissive and non-emissive displays, which may generally comprise multiple display elements. In order to control such displays, the current, voltage or any other physical parameter driving the display element may be manipulated. When appropriately driven, these display elements, such as pixels, normally develop light that can be perceived by viewers.
In an emissive display example, to drive a display (e.g., a display matrix having a set of pixels), electrical current is typically passed through selected pixels by applying a voltage to the corresponding rows and columns from drivers coupled to each row and column in some display architectures. An external controller circuit typically provides the necessary input power and data signal. The data signal is generally supplied to the column lines and is synchronized to the scanning of the row lines. When a particular row is selected, the column lines determine which pixels are lit. An output in the form of an image is thus displayed on the display by successively scanning through all the rows in a frame.
For instance, a spatial light modulator (SLM) uses an electric field to modulate the orientation of an LC material. By the selective modulation of the LC material, an electronic display may be produced. The orientation of the LC material affects the intensity of light going through the LC material. Therefore, by sandwiching the LC material between an electrode and a transparent top plate, the optical properties of the LC material may be modulated. In operation, by changing the voltage applied across the electrode and the transparent top plate, the LC material may produce different levels of intensity on the optical output, altering an image produced on a screen.
Typically, a SLM, such as a liquid crystal on silicon (LCOS) SLM, is a display device where a LC material is driven by circuitry located at each pixel. For example, when the LC material is driven, an analog pixel might represent the color value of the pixel with a voltage that is stored on a capacitor under the pixel. This voltage can then directly drive the LC material to produce different levels of intensity on the optical output. Digital pixel architectures store the value under the pixel in a digital fashion, e.g., via a memory device. In this case, it is not possible to directly drive the LC material with the digital information, i.e., there needs to be some conversion to an analog form that the LC material can use.
In field sequential display devices, multiple colors are multiplexed across a display device to achieve a full-color display. A color management system (e.g., a color wheel or other such mechanism) then illuminates the display panel with light of the appropriate color. For example, each video frame may be divided into three sub-frames that display red data, green data, and blue data in sequence. During each sub-frame, the display panel modulates according to the value of the color component being displayed while the color management system illuminates the panel with the appropriate color.
An approach used in field sequential devices is known as “scrolling”. In this approach, the data “scrolls” onto the display panel to improve efficiency. That is, rather than displaying all red data at the same time, the red data fills part of the display in time (as a result, the display panel will simultaneously display data from different color components), and so forth.
Scrolling systems can provide performance benefits in reduced display panel architectures. While analog modulation schemes are fairly easy to migrate to scrolling approaches, digital approaches face additional issues since they must properly transition state in the time domain. Thus scrolling presents certain challenges for digital modulation. A need thus exists to effectively implement digital modulation in scrolling and non-scrolling systems.